The present invention relates to a headlamp device for a vehicle, such as an automobile, in which an illumination optical axis is deflected by using a motor as a drive source. More particularly, the invention relates to a vehicular headlamp device well adaptable for a headlamp device which uses headlamps each provided with light distribution control means, for example, an AFS (adaptive front lighting system), for causing an illumination direction and an illumination range of the headlamp to change in accordance with a running state of the vehicle.
A technique described in U.S. Published patent application 2002-0064051 A1 (published on May 30, 2002) may be presented as an example of the AFS proposed as a technique for improving the running safety of the automobile. As shown in a conceptual diagram of FIG. 1, the AFS detects information representative of a running state of an automobile CAR by use of sensors 1, and outputs detecting output signals of the sensors to an ECU (electronic control unit) 2. Those sensors 1 are a steering sensor 1A, a speed sensor 1B, and car height sensors 1C. The steering sensor 1A detects a steering angle of a steering wheel SW of the automobile CAR, for example. The speed sensor 1B detects a speed of the automobile CAR. The car height sensors 1C detects heights of the front and the rear axles for detecting a leveling of the automobile CAR (only the vehicle height sensor attached to the rear axle is illustrated in the figure). Those sensors 1A, 1B and 1C are connected to the ECU 2. The ECU 2 receives output signals from the sensors 1, and controls swivel lamps 3R and 3L installed on the right and left locations of the front of the automobile in accordance with the received output signals. The swivel lamps are headlamps of which the light distribution characteristics may be changed by deflecting the illumination directions thereof to right and left. In an example of the swivel lamp 3R (3L), a reflector or a projector in the headlamp is designed to be rotatable in horizontal directions, and a rotation drive means drives the reflector or the projector to rotate by using a drive source, such as a drive motor. A mechanism including the rotation drive means will be referred to as an actuator. When an automobile runs on a curved road, this type of AFS enables the headlamps to provide frontal illumination ahead of the curve in accordance with a running speed of the automobile. Accordingly, the AFS is effective in improving the running safety of the automobile.
To produce a proper illumination in the AFS, it is necessary to properly control a rotational direction and a rotation quantity of the drive motor of the actuator. To the control, it is necessary to exactly detect a rotational position of the drive motor. For example, when the ignition switch of the automobile is turned on, an initializing process to set the optical axes of the headlamps at predetermined deflection positions is carried out. Specifically, the process detects a rotational direction and a rotation quantity of the motor that are measured with respect to a reference rotational position, and sets the rotational position of the motor at an initial position on the basis of the detected rotational direction and the rotation quantity. Therefore, some means to detect an rotational position of the motor is required.
A brushless motor in which a permanent magnet is used for the rotor is conventionally used for the drive motor in the AFS. Accordingly, an rotational position of the motor is detected by using Hall elements for detecting a variation of a magnetic field developed from the rotor. This will be described by using a brushless motor which will be discussed in an embodiment to be described later. As shown in FIG. 7, a rotary shaft 423 is rotatably supported by a first hollowed boss 414 being fixed. A cylindrical rotor 426 is fixedly mounted on the rotary shaft 423. The cylindrical rotor 426 is provided with an annular rotor magnet 428. The annular rotor magnet is mounted on an inner surface of a cylindrical yoke 427 made of synthetic resin, and is circumferentially magnetized to have S and N poles alternately arrayed. Within the cylindrical rotor 426, a stator coil 424 including three pairs of coils equidistantly arrayed in the circumferential direction is fixedly supported on a core base 425. Further, three Hall elements or Hall ICs (referred to as Hall elements) H1, H2 and H3 are arrayed at given angular intervals along the circumference of the cylindrical rotor 426. When the cylindrical rotor 426 is rotated, a magnetic field by the annular rotor magnet 428 varies at the Hall elements H1, H2 and H3. Responsive to the variation of the magnetic field, the Hall elements H1, H2 and H3 change their on/off states to produce pulse signals each having a rectangular waveform corresponding to a rotational period of the cylindrical rotor 426.
In the brushless motor, an AC having different phases U, V and W is supplied to the three coil pairs of the stator coil 424. Then, the directions of the magnetic forces between the coils and the annular rotor magnet 428 are changed to rotate the cylindrical rotor 426 and the rotary shaft 423. With the rotation of the cylindrical rotor 426, the Hall elements H1, H2 and H3 periodically produce rectangular signals (pulse signals). A rotation quantity of the motor may be detected by counting those pulse signals, and a rotational direction and an rotational position of the cylindrical rotor 426 may be detected by computing the logical values of the pulse signals. Another possible way to control the optical axis deflection is that an output angle of the actuator is detected and the radiation optical axis of the headlamp device is controlled in accordance with the detected output angle. In this case, a potentiometer for detecting the rotational position must be provided on the output shaft of the actuator. Provision of the potentiometer is not preferable since it will make the structure of the actuator complicated and increase the size of the actuator.
FIG. 12 is a waveform diagram showing waveforms of output signals of the Hall elements H1, H2 and H3. For example, as shown in FIG. 7, the rotor magnet has different magnetic poles arrayed at the center angle 180°, and the three Hall elements are arrayed at the center angle of 120°. In this case, when the rotor rotates by one turn, the pulse signals output from the Hall elements H1, H2 and H3 vary as shown in FIG. 12(a). As shown, each pulse signal has a periodic waveform in which H level and L level alternately appear with rotation of the motor. Accordingly, it is easy to detect a rotation quantity of the rotor in a manner that pulse signals of the Hall elements are detected, and the pulse signals are counted. If the pulse signals of the Hall elements H1, H2 and H3 are converted into binary signals of H level and L level, combinations as shown in FIG. 13(a) are obtained. Rotational positions arrayed every 60° and the rotational directions of the drive motor are detected from the combinations of those binary signals, i.e., 3-bit code values encoded.
If in such a drive motor, one of the three Hall elements becomes defective and is incapable of producing an output signal, it is difficult to detect an rotational position of the drive motor. Specifically, if the Hall element H1 becomes defective, and is incapable of producing an output signal, the pulse signals output from the Hall elements H2 and H3 vary as shown in FIG. 12(b). Accordingly, as shown in FIG. 13(b), the code value based on the binary signal remains unchanged in a range from the rotational positions P1 to P2 and a range from the rotational positions P4 to P5. Therefore, it is impossible to detect an rotational position located at a midpoint of each of those ranges. As a result, the rotational position of the motor cannot be detected exactly. The rotation control of the drive motor cannot be controlled properly. Further, the drive torque decreases, so that it is difficult to obtain a motor speed as intended or sometimes it is impossible to continuously control the rotation of the drive motor. Consequently, those facts hinder the exact control of the optical axis deflection in the AFS. Those problems are present not only in the Hall element but also in any other detecting element if it detects the rotation of the motor.